Field of the Invention
The invention concerns a method to determine a control sequence for a magnetic resonance imaging system, and a method to operate a magnetic resonance imaging system, a control sequence determination device to determine a control sequence for a magnetic resonance imaging system; and a magnetic resonance imaging system with such a control sequence determination device.
Description of the Prior Art
Magnetic resonance imaging procedures have found numerous usage, for example for medical applications as well as for non-destructive material and structural analysis of components and other examination subjects. In a magnetic resonance imaging system, for data acquisition a static basic magnetic field is typically overlaid with a targeted, spatially differing magnetic field (known as the gradient magnetic field). The basic magnetic field serves for the initial alignment and homogenization of magnetic dipoles (i.e. of rotational characteristics known as “spins”) at examined nuclei. The spatial resolution of the acquired magnetic resonance signal used for imaging takes place at least in part by the gradient magnetic field.
Depending on the operating mode of the imaging system, different switching sequences and magnetic field strengths for the gradient magnetic field are established that can be generated and varied by the gradient coil system. These switching sequences are predetermined in a control sequence that likewise provides control of an RF transmission system of the imaging system in a temporally coordinated manner to emit radio-frequency pulses in order to deflect the magnetic dipoles in the examination region out of the basic alignment. The gradient coil system to generate the gradient magnetic field is typically a rapidly switched, electrically operated coil system with multiple gradient coils that generate magnetic fields (for example in spatial directions x, y and z orthogonal to one another) in an established manner with the use of currents that are in the range of a few 100 amps.
Due to interaction forces (Lorentz forces) of these currents with the basic magnetic field of the tomography system and the interaction of magnetic scatter fields of the gradient coil system (eddy current forces) with conductive regions of the tomography system, strong mechanical oscillations of the gradient coil system occur that—in addition to a high stress to the tomography system in mechanical terms—lead to severe generation of perceptible noise.
The control sequence can be optimized with regard to the noise generation of the tomography system. A number of different approaches that affect a large number of parameters of a control sequence are known for this purpose.
One possibility to optimize the control sequence is described for “Fast Spin-Echo (RARE)” or “Fast Gradient Echo (FLASH—fast low angle shot)” control sequences in an article by F. Hennel, “Fast Spin Echo and Fast Gradient Echo MRI With Low Acoustic Noise”, Journal of Magnetic Resonance Imaging 13, 2001, P. 960 to P. 966. The underlying principles of the optimization are:                the gradient pulses of the control sequence have no plateau after the optimization, unless the selection of a different slice to be acquired takes place, and        the time for a phase coding is expanded to a readout time period in which magnetic resonance signals have already been acquired.        
Another possibility to improve the control sequence with regard to the noise generation is explained in the DE 198 14 950 A1, wherein the change of the strength of magnetic field gradients that are used for signal excitation and/or spatial coding follows a sigmoidal function whose second derivative has no maxima or minima. The use of multislice-selective pulses is necessary in order to be able to also apply this excitation sequence in “Echoplanar Imaging (EPI)” or “Echo shifting FLASH (ES-FLASH)” sequences.
In an article by P. Latta et al., “Single point imaging with suppressed sound pressure levels through gradient shape adjustment”, Journal of Magnetic Resonance 170 (2004), P. 177 through P. 183, it is also explained that the noise emission of the gradient system can be minimized via suitable selection of the parameters “gradient amplitude A”, “gradient plateau length P”, “ramp rise time R” and “sequence repetition time TR”.
An additional possibility to optimize a control sequence with regard to the noise exposure is disclosed in the U.S. Pat. No. 6,452,391 B1. The parameters “amplitude change rate”, “amplitude” and “pulse duration” of the control sequence are modified for optimization.
In addition to the optimization with regard to the noise emission, in the modification of the control sequence it is also advantageous to achieve an optimally precise agreement with a desired target magnetization. A method to optimize multichannel pulse trains to produce agreement with a target magnetization is known from DE 10 2010 033 329 A1. Limits of hardware operating parameters of the RF transmission device that serves to generate the target magnetization are taken into account in the optimization.
As the large number of proposals indicates, there continues to be a need to minimize the noise emission of a tomography system.